This invention relates to aluminous abrasive grits and particularly to sol-gel alumina abrasive materials with improved grinding performance.
Sol-gel alumina abrasives are conventionally produced by drying a sol or gel of an alpha alumina precursor, (which is usually but not essentially, boehmite), at about 125 to 200.degree. C. to remove the water component of the gel; breaking up the dried gel into particles of the desired size for abrasive grits; perhaps calcining the particles, (generally at a temperature of from about 400-800.degree. C.), to form an intermediate form of alumina; and then finally firing the calcined pieces at a temperature sufficiently high to convert them from an intermediate form such as gamma alumina to the alpha alumina form. Simple sol-gel processes are described for example in U.S. Pat. Nos. 4,314,827; 4,518,397; 4,881,951 and British Patent Application 2,099,012. Sol-gel aluminas of this type tend to have crystal sizes of up to 25 microns or more, though modifying additives such as silica, spinel formers such as magnesia, and other metal oxide additives such as zirconia, yttria, rare earth metal oxides, titania, transition metal oxides and the like have been used in minor amounts to reduce the crystal size to about 1 to 10 microns and enhance certain physical properties.
In a particularly desirable form of sol-gel process, the alpha alumina precursor is "seeded" with a material having the same crystal structure as, and lattice parameters as close as possible to, those of alpha alumina itself. The "seed" is added in as finely divided form as possible and is dispersed uniformly throughout the sol or gel. It can be added ab initio or it can be formed in situ. The function of the seed is to cause the transformation to the alpha form to occur uniformly throughout the precursor at a much lower temperature than is needed in the absence of the seed. This process produces a crystalline structure in which the individual crystals of alpha alumina, (that is those areas of substantially the same crystallographic orientation separated from adjacent crystals by high angle grain boundaries), are very uniform in size and are essentially all sub-micron in diameter for example from about 0.1 to about 0.5 micron in diameter. Suitable seeds include alpha alumina itself but also other compounds such as alpha ferric oxide, chromium suboxide, nickel titanate and a plurality of other compounds that have lattice parameters sufficiently similar to those of alpha alumina to be effective to cause the generation of alpha alumina from a precursor at a temperature below that at which the conversion normally occurs in the absence of such seed. Examples of such seeded sol-gel processes are described in U.S. Pat. Nos. 4,623,364; 4,744,802; 4,954,462; 4,964,883; 5,192,339; 5,215,551; 5,219,806 and many others.
The optional calcining of the dried sol-gel is often preferred so as to minimize the time needed at the elevated firing temperatures. This is because the firing operation performs the tasks of converting the transitional alumina forms to the alpha form and the sintering of the alpha alumina to close up residual porosity and ensure that the particles have adequate density and hardness to function well as abrasive grits. It is known that excessive time at sintering temperatures, which are generally between 1300 and 1400.degree. C. for seeded sol-gel materials and about 100.degree. C. higher than that for unseeded sol-gel aluminas, can lead to crystal growth. Since crystal growth is generally regarded as undesirable, it is considered appropriate to carry out the calcining separately and so minimize the time at such elevated temperatures. This procedure is followed in spite of the extra cost of maintaining two high temperature operations.
Since the drying operation is followed by a crushing and screening operation the grain is reduced to room temperature and the heat used to dry the grain is given up to the surroundings. This is of course very inefficient.
The crushing operation is performed after the drying because, at this point the material is relatively easily broken up. If it were left till after the firing operation, the material would be so hard that excessive amounts of energy would be required. It is therefore common-sense to crush at the pre-fired stage. In addition it is considered that firing will be more efficient since the particles will more rapidly reach the firing temperature in the furnace if they are small.
It has now been found possible to significantly reduce the energy consumption involved in the production of alumina by a sol-gel process. This is achieved by a manipulation of the process in a manner that is completely contrary to the intuitive reasoning used in designing conventional processes. The novel process produces alpha alumina particles in a very desirable form that is fully densified and well adapted to use in abrasive applications. Moreover the system is flexible enough to permit design of the abrasive grits obtained.